This invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx. More particularly, the invention pertains to an improved non-thermal plasma reactor and system for use with diesel engines and the like.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz.
An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective.
A reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/268,496 entitled xe2x80x9cNon-thermal Plasma Exhaust NOx Reactor.xe2x80x9d Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field
There remains a need in the art to minimize the potential failure condition whereby high voltage could leak away from the reactor due to the short distance between the electrode and the connector. There further remains a need for an improved reactor that can offer the structural integrity of current automotive catalysts while providing improved high voltage control.
The present invention provides a monolithic non-thermal plasma reactor (MPR) in which the electrodes may be silver or platinum material coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates. This prevents arcing while providing stable electrode spacing needed for a uniform electric field.
Opposite polarity plates are retained by a dielectric structure, such as a honeycomb extrudate common in today""s automotive catalytic converters. The extrudate may be coated or can be made of a catalytic material whose structure and chemical composition provide active sites for NOx reduction. Alternatively, a downstream catalytic converter may be provided in the system to complete the conversion of NOx to nitrogen and oxygen. The use of the catalyst significantly reduces the power required to complete the conversion of NOx.
In the MPR, each of the parallel plate pairs forms a cell. These cells are stacked in order to subject the exhaust gases to as much catalyst surface in plasma as is possible within a minimal volume. By reducing the electrode spacing of the cells, lower voltages can be used to create a corona. This minimizes the extraordinary safeguards and barriers needed to contain very high (15,000 plus) voltages of the packed bed reactors, proving a simpler and lower cost electrical delivery system.
The use of electrode coated plates may be made by the use of ceramic inks and printing processes which provides the simplicity of making connections and conductors through conventional screening processes. This eliminates the need for wires and connectors and their associated assembly complexity and unreliability.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.